1. Field of the Invention
The present invention is directed to a method in the form of a pulse sequence for operating a nuclear magnetic resonance tomography apparatus.
2. Description of the Prior Art
A pulse sequence is known from Journal of Magnetic Resonance, Series B 101 (1993), February, No. 1, pp. 106-109 wherein, during a first phase, a sequence of hard radio-frequency pulses is emitted and a gradient in a first direction is simultaneously activated and, during a third phase, the gradient in the first direction is inverted and the nuclear magnetic resonance signals arising in a sequence of sampling intervals are sampled under the influence of the inverse gradient. The first and third phases are repeated n times in order to obtain a complete image.
Short image exposure times are thereby of special significance. This is true not only in view of an optimally high patient throughput but, for example, in order to avoid motion artifacts. Short image exposure times are unavoidable for certain exposure techniques, for example cine mode (registering moving images).
Fast pulse sequences have special significance, for example, for determining the spread of contrast agents, for observing heart motion, for the brain function and for the kinematics of joints since a plurality of images must be registered in fast succession.
Of the methods that have been hitherto disclosed, the shortest image exposure times (40 through 100 ms) can be achieved with the EPI method. The EPI method is disclosed, for example, by European Letters Patent 0 076 054.
At the beginning of the pulse sequence, an RF excitation pulse is thereby beamed onto an examination subject under the influence of a slice selection gradient in a first direction. Nuclear spins are thereby excited in a slice of the examination subject. After the excitation, a phase-encoding gradient is activated in a second direction and a readout gradient is activated in a third direction. First, second and third direction reside perpendicular to one another. The readout gradient is composed of a pre-phasing pulse as well as of sub-pulses of alternating polarity. Due to this alternating polarity of the readout gradient, the nuclear spins are dephased and rephased in alternation. After a single excitation, so many signals are thereby acquired that the entire k-space is scanned, i.e. that the existing information suffice for the reconstruction of a complete tomogram.
The phase-encoding gradient is briefly activated with every change in the polarity of the readout gradient. The phase position of the nuclear spins is thus advanced by one step each time. The arising nuclear magnetic resonance signals are sampled in the time domain, digitized, and the numerical values acquired in this way are entered into a raw data matrix. An image of the examination subject is then reconstructed from this raw data matrix on the basis of a two-dimensional Fourier transformation. The speed advantage of the EPI method is essentially based thereon that a plurality of signals are acquired after a single excitation, these signals sufficing for the reconstruction of a complete tomogram. All signals that ultimately represent gradient echoes must be acquired within the T2* decay. The readout gradient must therefore be very rapidly bipolarly switched, so that considerably technological demands are made of the system.
Further, gradient echoes as generated in the EPI method have the disadvantage compared to spin echoes that they are sensitive to local field inhomogeneities.
U.S. Pat. No. 4,818,940 discloses a pulse sequence wherein a plurality of spin echoes are acquired due to a plurality of successive 180.degree. radiofrequency pulses after a 90.degree. radiofrequency pulse. The exposure times in this method, however, are longer than in the EPI method and the permitted radiofrequency stress on the patient is soon reached given fast repetition.
U.S. Pat. No. 5,126,673 discloses a pulse sequence wherein a sequence of many equidistant radiofrequency pulses, what is referred to as a pulse burst, is beamed in for the excitation of a specimen. The radiofrequency pulses comprise an extremely small flip angle on the order of magnitude of 0.1.degree. through 2.degree.. A train of equidistant echo signals with optimally constant amplitude is obtained following the sequence of the radiofrequency pulse. Amplitude and phase of the radiofrequency pulses are influenced in order to keep the amplitude of the echo signals optimally constant. A selective excitation or, refocussing as well as readout and phase-encoding gradients are provided for the imaging.
The possibility of omitting a more or less large plurality of radiofrequency pulses from the sequence of radiofrequency pulses is also mentioned. The disclosed, non-equidistant radiofrequency pulse sequence has the disadvantage that the amplitude constancy of the echo signals is difficult to optimize. Further, a plurality of echoes respectively coincide in the readout phase, so that a clean evaluation for image acquisition is practically impossible. Moreover, the signal-to-noise ratio becomes very unfavorable due to the small flip angles in the excitation.
U.S. Pat. No. 5,212,448 discloses a pulse sequence wherein a series of excitation radiofrequency pulses is respectively beamed in under slice-selection gradients. The excited spins are refocussed in chronological succession by subsequent application of a radiofrequency inversion pulse.
The article "QUEST--A Quick Echo Split Imaging Technique" in Book of Abstract, page 433, Annual Meeting of the Society of Magnetic Resonance and Medicine, 1992, likewise discloses a pulse sequence wherein a sequence of temporally non-equidistant excitation pulses and a first gradient are activated during an excitation phase. The existing spin magnetization is split into sub-collectives due to each excitation pulse following the first excitation pulse. During the readout phase, a temporally graduated refocussing of the individual sub-collectives ensues under a further gradient. A plurality of nuclear magnetic resonance signals can thereby be acquired after a single excitation phase, so that a short image exposure time is possible. The demands made of the gradient electronics remain low. This method, however, has the disadvantage that the acquired signals are difficult to interpret due to the increasing splitting into sub-collectives.